Centrifugal impellers are used in turbine machines, such as a centrifugal pump or a centrifugal compressor, to provide high pressure working fluid to a combustor. In some turbine machines, for example, a centrifugal compressor may be used as the final stage in a multi-stage high-pressure gas generator.
FIG. 1 is a simplified cross-sectional view of a centrifugal compressor 100 in a gas turbine engine. The centrifugal compressor 100 comprises a centrifugal impeller 111 and shroud 120. The centrifugal impeller 111 is mounted to a rotatable shaft 116 and has a plurality of blades 112 coupled thereto. The blade 112 extends between an inducer 115 and an exducer 117. The radially-outward surface of each of the plurality of compressor blades 112 comprises a compressor blade tip 113. As the centrifugal impeller 111 rotates, it receives working fluid at a first pressure and ejects working fluid at a second pressure that is higher than the first pressure.
An annular shroud 120 encases the plurality of blades 112 of the centrifugal impeller 111. The gap between a radially inner surface 122 of shroud 120 and the impeller blade tips 113 is the blade tip clearance 140 or clearance gap. Shroud 120 may be coupled to a static portion of the engine casing 131 directly or via a first mounting flange 133 and second mounting flange 135.
Gas turbine engines having centrifugal compressors 100 such as that illustrated in FIG. 1 typically have a blade tip clearance 140 between the blade tips 113 and the shroud 120 set such that a rub between the blade tips 113 and the shroud 120 will not occur at the operating conditions that cause the greatest closure of the blade tip clearance 140. A rub is any impingement of the blade tips 113 on the shroud 120. However, setting the blade tip clearance 140 to avoid blade 112 impingement on the shroud 120 during transients having the greatest closure of the blade tip clearance 140 may result in a less efficient centrifugal compressor 100 because working fluid is able to flow between the blades 112 and shroud 120, thus bypassing the blades 112 by flowing through gap 140. This constitutes leakage. In the centrifugal compressor 100 of FIG. 1, blade tip clearances 140 cannot be adjusted because the shroud 120 is rigidly mounted to the engine casing 131.
It is known in the art to dynamically change blade tip clearance 140 to minimize rub while also reducing leakage of the working fluid around the blade tips 113. Several actuation systems for adjusting blade tip clearance 140 during engine operation have been developed. These systems often include complicated linkages, contribute significant weight, and/or require a significant amount of power to operate. Thus, there continues to be a demand for advancements in blade clearance technology to minimize rub while avoiding leakage.